1. Field of the Invention
The present invention relates generally to circuit substrates having a printed circuit board on which electronic devices generating much heat and requiring cooling (hereinafter referred to as heat-generating electronic devices) are mounted, and more particularly, to a circuit substrate which permits the influence of heat from the heat-generating devices upon the entire printed circuit board and other peripheral electronic devices by localizing the conduction of the heat to the mounted area of the electronic devices or to a particular direction, thereby restricting the conduction of heat in the direction along the surface of the printed circuit board.
2. Description of the Background Art
Devices such as CPUs installed in electronics such as work station and personal computer generate significant heat. Thus, the process of radiating heat from such a heat-generating electronic device is indispensable in order to provide satisfactory operation specs.
In conventional devices, in order to radiate heat generated from a heat-generating electronic device installed on a top surface layer of a printed circuit board, there are provided a number of through holes at the printed circuit board, and the through holes are plated. Then, the heat-generating electronic device and the through holes are connected. Further by connecting the through holes at the back surface layer of the printed circuit board to a heat radiation plate, heat generated from the electronic device is allowed to escape onto the heat radiation plate through the printed circuit board via these through holes.
In the conventional structure, however, a significant amount of heat is still transmitted from the heat-generating electronic device to the printed circuit board, the heat is transmitted in the vertical direction from the top surface layer to the back surface layer of the circuit board, in other words in the thickwise direction of the substrate, but part of the heat is also diffused in the crosswise direction of the printed circuit board, in other words in the direction along the surface, and the heat generated from the mounted portion of the printed circuit board adversely affects other peripheral electronic devices. Furthermore, the temperature of solder generally used as means for connecting electronic devices is raised at soldered parts of the printed circuit board by the conduction of heat, which could adversely affect the entire circuit substrate and is likely to cause operation faults or shorten the useful life of the circuit substrate.
FIGS. 1A and 1B are a plan view and a cross sectional view, respectively of a circuit substrate in a first conventional example disclosed by Japanese Patent Publication No. 5-52079. The circuit substrate shown in FIGS. 1A and 1B is incorporated into a personal computer or the like, and an electronic device generating much heat (heat-generating electronic device) 22 is provided on a printed circuit board 21.
Printed circuit board 21 includes a top surface layer 21a, a bottom surface layer 21b, a pair of inner layers 21c provided between top and bottom surface layers 21a and 21b and serving as a signal layer having signal lines for a particular circuit, and a ground plane layer 21d provided between the pair of inner layers 21c. There is provided at the top surface layer 21a or bottom surface layer 21b of printed circuit board 21, a die attaching pad 23 serving as a mounting portion electrically and physically insulating within circuit board 21.
Note that a denotes the die area.
Heat-generating electronic device 22 takes the form of TCP (Tape Carry Package), and the central portion of heat generation (die) 24 is secured to the die area a of die attaching pad 23 on the top surface layer 21a of the printed circuit board by a thermally conductive adhesive, a die attaching adhesive 25. Printed circuit board 21 having heat-generating electronic device 22 mounted thereon has its back surface side connected to a heat radiation plate 27 through solder or a thermal compound 26.
Thus, in the first conventional example, one or more plated through holes 28 are provided at printed circuit board 21, heat from heat-generating electronic device 22 is transmitted onto the back surface layer 21b of printed circuit board 21 through through holes 28, and the heat is further transmitted to heat-radiation plate 27.
FIGS. 2A and 2B are a plan view and a cross sectional view, respectively of a circuit substrate in a second conventional example disclosed by Japanese Patent Publication No. 5-52079. In the second conventional example, one or more relatively large, un-plated through holes 29 are provided at printed circuit board 21.
Through hole 29 is formed larger than die area a, and a fitting portion 27a extended as a projection from heat-radiation plate 27 is fitted therein. Heat-generating electronic device 22 is directly connected to a surface of radiation plate fitting portion 27a exposed from through hole 29 using die attaching adhesive 25. As a result, heat from heat-generating electronic device 22 may be directly transmitted to radiation plate 27 without being passed through printed circuit board 21.
FIG. 3 shows a third conventional example, in which only heat-generating electronic device 22 is separately provided at a dedicated printed circuit board 21, so that heat generated from heat-generating electronic device 22 will not be transmitted to other peripheral electronic devices 30, 31 and 32. These other peripheral electronic devices 30, 31 and 32 are mounted on a printed circuit board 21a which is separately prepared, and a stacking connector 33 or the like is provided to connect these printed circuit boards 21 and 21a.
The first to third conventional examples are encountered with the following problems. In the third conventional example, printed circuit board 21 dedicated to heat-generating electronic device 22 should be provided separately from printed circuit board 21a for other peripheral electronic devices, which is disadvantageous in terms of cost for materials as well as in terms of mounting efficiency and electrical characteristics as compared to the case of mounting heat-generating electronic device 22 and other devices together at a single printed circuit board 21.
In contrast, the first conventional example is advantageous in terms of the efficiency, because heat generated from electronic device 22 is transmitted from the top surface layer 21a of printed circuit board 21 to bottom surface layer 21b, and then onto heat-radiation plate 27. However, since the heat conduction is through a number of through holes 28 provided with plated layers at printed circuit board 21, and therefore the part of printed circuit board 21 having through holes 28 is exposed to high heat from heat-generating electronic device 22.
As a result, as heat diffuses onto the entire surface of printed circuit board 21, the temperature of printed circuit board 21 is raised, and thermal stress imposed on the other peripheral electronic devices or the soldered portion cannot be avoided.
In the second conventional example, heat radiation plate fitting portion 27a is engaged into one or more relatively large un-plated through holes 29 provided at printed circuit board 21, and therefore fitting portion 27a and heat-generating electronic device 22 are directly connected. Thus, heat conduction is allowed without transmitting heat from heat-generating electronic device 22 to printed circuit board 21. However, die attaching adhesive 25, a generally heat conductive adhesive, is used as means for connecting the surfaces of heat-generating electronic device 22 and heat radiation plate 27, and therefore die attaching adhesive 25 should be removed and re-adhesion is necessary during disassembling and replacement working for repairing or the like.
Since the adhesion process by die attaching adhesive 25 requires a heat curing process, and therefore after heat-generating electronic device 22 is again attached by applying die attaching adhesive 25, a heat-curing process should be once again performed.
When heat is radiated through printed circuit board 21, in general heat radiation structures including the above conventional examples, heat from heat-generating electronic device 22 is diffused into the periphery of printed circuit board 21 or printed circuit board 21 is shielded against heat. Therefore, the direction of heat conduction and diffusion onto printed circuit board 21 cannot be specified.
As described above, the first and second conventional examples both suffer from the problems, and to achieve optimum heat radiation for printed circuit board 21, it is inevitable to transmit heat from heat-generating electronic device 2 to the side of bottom surface layer 21b through a number of plated through-holes, shield printed circuit board 21 against heat conduction and diffusion, whereby to avoid transmission of heat onto other electronic devices 30, 31, and 32 mounted on printed circuit board 21 and the soldered portions.